Quoting Robert Hentschke,
There's no need to discuss the practical details, I'm just putting the idea out there to see what people think of it in general terms.Obviously, this would only apply to total conversion mods (think FallFromHeaven2), rather than mods that tweak a few things here and there, and add a couple of techs, but don't change the core game.
I guess you're talking about total conversion mods that have no relation whatsoever to copyrighted material like LOTR, MoM, or generally any books or movies...

I was actually talking about the principle of the whole thing.
Copyright is a whole other issue.
Scenario: I spend all my free time over the next year making a game/mod. I'm quite pleased with the result. I'd like people to try it. At the moment, I don't know anybody else who's bought Ele.... they're all getting Civ instead. And asking them to spend £40 on a game just to try my mod, well, that's a bit daft.
Ideally, I'd like to give it away for free, to see what they think. But I can't do this if I use Ele's engine.
This isn't an ideal situation. And Civ5 is going to sell a huge amount, most my friends are buying it, so maybe i'd be better modding that. Even if it's tools aren't as great. As at least then I can give people copies!
Or maybe I'm best just playing around trying to make a tbs game inside Unity3D.
(Haven't many progress in quite a while though)